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The use of intense acoustic waves to hold a body that is immersed in a fluid medium against the force of gravity without obvious mechanical support.
Levitation can occur in the presence of fluid flow, including the back-and-forth fluid flow produced by the passage of an acoustic wave. Such acoustically generated forces are extremely small in common experience. But intense acoustic waves are nonlinear in their basic character and, therefore, may exert a net acoustic radiation pressure on an object sufficient to balance the gravitational force and thus levitate it. See Acoustic radiation pressure
The applications of acoustic levitation in air or other gas include an acoustic positioning module that has been carried in the space shuttle and used in fundamental studies of the oscillation and fission of spinning drops. An acoustic levitation furnace has been designed to study the possibility of containerless solidification of molten materials.
Applications of the levitation of objects in liquids have included measurements of the ultimate tensile strengths of liquids, mechanical characterization of superheated and supercooled liquids, the measurement of properties of biological materials (including human red blood cells and lipids from the porpoise dome), the study of shape oscillations and interfacial tension of levitated drops, and the evaporation of charged drop arrays levitated electroacoustically. See Sound